Since their first use as quasiparticle mixers, SIS (superconductor-insulator-superconductor) tunnel junctions have become the lowest noise broad band mixers from 30 GHz to 760 GHz. Sensitivity of these mixers has approached the quantum limit (h.nu. power per unit bandwidth) at frequencies up to 110 GHz. A comprehensive theory of these devices has been developed which predicts excellent performance for SIS tunnel junctions as mixers well below millimeter wavelengths. However, actual development of submillimeter SIS mixers has been limited by currently used lead alloy junctions. Lead alloy junctions give excellent mixer results in the near-millimeter wave region, but have poor chemical, thermal, and electrical stability because lead is a soft, low melting point material.
For high speed switching or high frequency detection applications utilizing SIS tunnel junctions, devices with very thin tunnel barriers and correspondingly high current densities (J.sub.c) are required. This constraint arises from the fact that the junction resistance decreases exponentially with decreasing barrier thickness, while the capacitance only increases linearly as the barrier thickness is reduced. Thus, the maximum operating frequency, which is inversely proportional to the resistance-capacitance (RC) product, can be increased by reducing the tunnel barrier thickness. Practical applications also require that the junction resistance be large enough (typically .gtoreq.50 .OMEGA.) to allow impedance matching to external circuits. Because the RC product is independent of area while the resistance increases as the junction area is decreased, this requirement can be met by fabricating small cross-sectional area tunnel junctions. To obtain junctions with adequate resistances and frequency response above 200 to 300 GHz, it is necessary to produce junctions with areas less than approximately 1.0 .mu.m.sup.2, which are difficult to achieve using conventional photolithography.
One useful technique for fabrication of small area tunnel junctions utilizes an edge geometry to achieve very small junction areas without resorting to high resolution lithography. Superconducting edge junctions have been fabricated in a variety of materials systems, including Pb/Sn, PbIn/Pb, Nb/Pb-alloys, and NbN/Pb. However, relatively little work exists on all-refractory edge junctions, with only brief reports on Nb/Nb.sub.2 O.sub.5 /Nb and NbN/Si/NbN junctions.
A recent patent by J. J. Talvaccio et al (U.S. Pat. No. 4,768,069) discloses a planar geometry of NbN/MgO-CaO/NbN. The substrate is heated to about 700.degree. C. to sputter-deposit the first NbN electrode and the oxide barrier layer.
For most practical high frequency applications, it is also necessary to fabricate superconducting tunnel junctions with high quality current-voltage (I-V) characteristics. In this case "high quality" means low subgap leakage coupled with a current rise at the gap sum which is narrow on the scale of the operating frequency. It is also highly desirable to utilize junction materials which are resistant to chemical attack and static discharges. In summary, then, high frequency applications require rugged, small area tunnel junctions with high current density, high quality I-V characteristics. Such characteristics are not achieved with the prior art teachings.